One of solutions based on the concept of information life cycle management is a storage hierarchization technology. This technology is to classify storage units where information is to be stored according to tiers based on the performance, cost, and reliability in accordance with utility value of the information. For example, storage units are generally hierarchized into three tiers, that is, Tier 1, Tier 2, and Tier 3.
Tier 1 is a storage unit for storing information about the highest information value, which is created and currently utilized. A Fibre Channel storage unit which demonstrates high-performance and is highly reliable is used as Tier 1. Tier 1 is also called an online storage unit.
Tier 2 is used to store data which is no longer utilized but is searched and referred to for reuse. A high-capacity, low-cost storage unit such as an ATA storage unit or an SATA storage unit is used as Tier 2.
Tier 3 is a storage unit specialized for storing data which is seldom accessed, and a tape drive whose cost per capacity is low is used. Tier 3 is also called an offline storage unit, and Tier 2, which ranks between Tier 1 and Tier 3, is called a nearline storage. An example of a conventional technique related to this invention is Japanese Unexamined Patent Application Publication No. 2008-305288.